This invention relates to an apparatus and method for optically pumping double-clad fiber lasers and amplifiers using diode lasers, diode bars, or fiber-coupled diode sources. More particularly, the invention relates to an apparatus and method for optical side-pumping an optical fiber that includes an embedded side-mirror.
Double-Clad Fiber Optical Sources
Rare-earth-doped fiber lasers and amplifiers are finding widespread use in applications requiring compact, rugged optical sources. In these sources, a rare-earth ion (e.g., Er3+ or Yb3+) is doped into the fiber core and is optically excited (typically using a diode laser as a pump source); a signal beam propagating in the core experiences gain if a population inversion has been established by absorption of the pump beam by the rare-earth ions (and if the signal beam has a wavelength within the gain spectrum of the rare-earth dopant). The core often supports only the lowest-order transverse mode of the signal beam (a single-mode (SM) fiber), but in some cases can support more than one transverse mode (a multimode (MM) fiber).
In a conventional, single-clad fiber, the signal and pump beams both propagate in the fiber core, which is surrounded by a cladding whose refractive index is lower than that of the core (thereby serving to define the size and numerical aperture, NA, of the core). In double-clad (DC) fiber, a second cladding with a lower refractive index (typically a fluorinated polymer) surrounds the cladding; the first or xe2x80x9cinnerxe2x80x9d cladding can thus guide light launched into it (much as the core guides light). In such a fiber, the signal beam is launched into the core (as in a single-clad fiber); the pump light, however, is launched into the much larger (and usually higher-NA) inner cladding. If the rare-earth dopant is confined to the core of the DC fiber, the pump light will be absorbed in the core, and signal light propagating in the core will experience gain, in a manner similar to that of a single-clad fiber. The advantage of DC fiber, however, is that it permits the use of pump sources that are relatively large (i.e., multimode), high-power, and inexpensive (in comparison with the single-mode pump sources capable of being launched into the core of single-clad fibers). The advent of DC fibers has allowed fiber sources to be scaled to average powers of  greater than 100 W.
Prior Art Coupling Schemes
Several techniques for launching pump light into DC fiber exist. These include end pumping, which is the most straightforward approach and which is often used in laboratory applications. The pump light is simply launched into the end of the fiber, typically using one or two lenses and possibly a mirror; optionally, pump light may be launched into both fiber ends. A major drawback of this technique is that one or both of the fiber ends are obstructed by the optics used to launch the pump light. Hence, coupling a signal beam into or out of the fiber requires a means to separate the pump and the signal (typically a dichroic mirror). In addition, this approach lacks scalability (the fiber has only two ends) and is difficult to implement in a compact and rugged manner; the simplicity is generally inferior to other techniques (larger parts count, more optical and mounting hardware). Finally, the fiber ends are easily damaged when high pump powers are used, e.g., if the fiber face is not kept very clean or if the pump beam becomes misaligned. In one implementation of end pumping (U.S. Pat. No. 5,185,758), multiple pump diodes, each with its own collimating lens, are arrayed along a focusing lens to provide more pump power (although not more brightness, because of the angular displacement of the beam from each diode impinging on the fiber face).
In another approach termed tapered, fused fiber bundles, which is described in U.S. Pat. No. 5,864,644, several diode lasers are coupled into individual multimode fibers; these fibers are bundled together, fused and drawn into a taper, and then fusion spliced to a DC fiber. Pump light from the diode lasers is thereby coupled into the inner cladding of the DC fiber. Optionally, the fiber bundle can include a single-mode fiber that is used to couple signal light into or out of the core of the DC fiber. This approach is stable and rugged (because the fibers are fused) and can have high efficiency (if the coupling efficiencies to the pigtails of the individual diode lasers are high). The approach is scalable, although it would be awkward to use with a diode bar. The problem of blocking the fiber end(s) is alleviated by employing the embodiment that incorporates a single-mode fiber into the bundle. Fabrication of a tapered, fused fiber bundle is a complex process, involving stripping the jacket (which exposes the delicate fibers), bundling the fibers in a close-packed formation, fusing (melting) the bundle, drawing the taper, and (usually) recoating with a low-index polymer. The shape and size of the fiber bundle must be customized for the given DC fiber being pumped.
In another approach termed V-groove side-pumping, described in U.S. Pat. No. 5,854,865 which is incorporated herein by reference, a V-shaped notch or groove is cut into the side of the DC fiber, and light from a pump diode (or fiber-coupled pump diode) is launched into the inner cladding by reflection from the facet of the V-groove. The depth of the V-groove is such that it penetrates the inner cladding but does not intersect the core. In the embodiment most commonly used, shown in FIG. 1a, a pump source 1 is placed on the opposite side of a fiber 2 from a V-groove 3, while a micro 4 is used to substantially focus the light onto a V-groove facet 5, and the pump light is coupled into an inner cladding 6 via total internal reflection from the facet. An outer cladding and jacket 7 are stripped from fiber 2 prior to cutting V-groove 3, and fiber 2 is mounted on a glass substrate 8 that transmits the pump light; the adhesive used for mounting fiber 2 to substrate 8 must have a refractive index less than or equal to that of the low-index fluorinated polymer of outer cladding 7 so that the light guiding properties of inner cladding 6 are not compromised. The angular acceptance of V-groove 3 can be increased by coating groove 3 for high reflectivity (HR), although this approach significantly increases the complexity of fabrication and subsequent servicing or repair.
Advantages of V-groove side pumping include high coupling efficiency and compact packaging; it is scalable (by cutting multiple V-grooves) and leaves the fiber ends unobstructed. In practice, however, this technique is alignment sensitive and thus presently lacks adequate long-term stability for many applications. The alignment sensitivity arises in part from the use of the lens (see below), which demagnifies the pump beam and thus increases its angular spread; moreover, available lenses do not allow the brightness of the pump source to be preserved. For a given combination of pump diode and DC fiber, a micro-lens has to be selected or fabricated to be compatible with the size and divergence of the diode and with the size and NA of the fiber inner cladding. For high-power applications, use of a diode bar as a pump source (rather than multiple diodes) would be desirable (see below). The prior art V-groove pumping technique, however, is not compatible with the use of diode bars because each element of the bar would require its own, individually aligned micro-lens, thus introducing prohibitive complexity; in principle, a lens array could be used, but present tolerances on the position and angle of the emitters on a diode bar are insufficient for this approach to be practical.
An alternative embodiment of V-groove side pumping that was recently introduced, in which the micro-lens is omitted, is described in Proceedings of the Conference on Lasers and Electro-Optics, OSA Technical Digest Series, paper CFC1 (Optical Society of America, Washington D.C., 2000), by L. Goldberg, J. Pinto, M. Dennis, and J. Koplow. Three possible configurations are shown in FIGS. 1b-1d. The approach shown in FIG. 1b is the easiest to implement (it is very similar to the usual embodiment shown in FIG. 1a, but without the lens), but the relatively large distance between pump diode 1 and V-groove 3 results in significant loss of brightness and potentially reduces the coupling efficiency. The approaches shown in FIGS. 1c and 1d reduce these problems. The configuration shown in FIG. 1c requires that V-groove 3 be HR coated, however, introducing substantial complexity in its fabrication; in addition, a reflection loss on the order of 4% would occur on facet 5 of V-groove 3 through which the pump light enters fiber 2 (the loss could be significantly higher if some of the HR coating were inadvertently deposited on this facet). The configuration shown in FIG. 1d requires that fiber 2 be rotated about the longitudinal axis by 180xc2x0 and bonded to substrate 8 after cutting V-groove 3, again introducing substantial complexity (the stripped fiber is very delicate, especially after the V-groove is cut). The approach shown in FIG. 1d has been demonstrated experimentally. Although the pump coupling efficiency was approximately 7% less than that obtained with the usual V-groove technique, more importantly, the sensitivity of the coupling efficiency to misalignment was reduced by a factor of 10 in comparison to the approach using a micro-lens, demonstrating the advantage of a xe2x80x9clens-lessxe2x80x9d coupling scheme.
In yet another approach, termed high-reflectivity coated fiber and described in U.S. Pat. No. 5,170,458, the fiber cladding is HR coated (effectively providing a DC fiber with a very high-NA inner cladding). Pump light is launched through a gap or xe2x80x9cwindowxe2x80x9d in the coating. This approach has not found widespread use.
Other approaches to coupling light into or out of an optical fiber are disclosed in U.S. Pat. Nos. 5,037,172 and 5,163,113, both of which involve coupling to the core (not the inner cladding). In the former patent, a V-groove that partially intersects the core is coated to increase the reflectivity of one face, thereby allowing a portion of the light propagating in the core to be coupled out of the fiber and/or allowing light to be coupled into the core. In the latter patent, the end of the fiber is cut at an angle to allow a light source on the other side of the fiber to be launched into the core by total internal reflection (xe2x80x9chalf of a V-groovexe2x80x9d). If adapted to pumping DC fibers, this approach would not offer significant advantages over V-groove side pumping; moreover, the technique is not scalable (the fiber has only two ends), and the fiber ends are obstructed by the mounting assembly.
The prior art approaches that involve applying an HR coating to a surface of a notch or groove introduced into a double clad fiber have significant disadvantages. Any coating technique proposed to date has proven to be cumbersome from the standpoint of manufacturing, relatively expensive, and thus impractical. The process conditions, e.g. the temperature at which coating is conducted, are harsh and threaten the structural integrity of the fiber itself, e.g. the outer cladding. Outgassing can occur, undesirably contaminating the coating being applied. Also, in the coating process, the coating undesirably collects on other surfaces such as the vertical face of the groove, reducing its light transmission efficiency. The process is irreversible once such damage occurs, although some of the fiber can usually be salvaged. For these and other reasons, the embodiments incorporating HR-coated grooves have not been carried out in practice or commercially to any extent.
An optical fiber includes a core doped with a preselected gain material, an inner cladding disposed about the core, an outer cladding, disposed about the inner cladding, that has a section removed to expose a portion of the inner cladding, a notch disposed in the exposed portion of the inner cladding, and a mirror disposed within the notch surface oriented so as to reflect light launched into the mirror from an outside source into the fiber, along the fiber axis. The mirror, e.g. a right-angle trihedron, has an HR coating on one face, is inserted into the notch, and affixed thereto with an adhesive such as an optical cement. Pump light, e.g. from a laser diode, is launched through a face of the mirror that is antireflection (AR) coated or sufficiently transparent to the light frequency, and after reflecting off the HR face exits through a third face and into the fiber. The pump light source can typically be positioned in close proximity to the input face of the mirror since there are no intervening optics or other hardware required in a typical system design.
According to another aspect of the invention, an optical fiber system for transmitting light includes the optical fiber with the embedded side-mirror and the diode.
According to another aspect of the invention, a method for launching light into an optical fiber includes the steps of providing the optical fiber, introducing the notch in the exposed portion of the inner cladding, disposing the mirror within the notch, and launching a light toward the mirror, whereby the mirror reflects the light into the inner cladding and the light is thereby transmitted along the fiber axis.
In a DC fiber source, the pump light must be launched into the inner cladding. As discussed above regarding the background of the invention, several launching methods have been developed. The present invention discloses a new launching method that possesses a number of distinct advantages. Several considerations affect the suitability and desirability of a pumping scheme:
Coupling efficiencyxe2x80x94the fraction of the pump light that is coupled into the inner cladding from the pump source (influences both the optical and the electrical efficiency of the system);
Stabilityxe2x80x94the short- and long-term variations in the coupling efficiency, which influence the sensitivity to mechanical and thermal disturbances and hence system reliability and ruggedness;
Compactnessxe2x80x94the size of the required components, mounting hardware, etc.;
Simplicityxe2x80x94the parts count, the required hardware, the alignment procedures, etc., which determine the complexity and the practicality of the coupling scheme (often by influencing the stability, compactness, and ruggedness);
Alignment sensitivityxe2x80x94affects the stability and the ease of implementation of the coupling scheme (alignment sensitivity should be minimized);
Scalabilityxe2x80x94the ability to scale up the power by using larger, more powerful pump sources or by using multiple pump sources (including diode bars);
Conservation of brightnessxe2x80x94the ability of the coupling scheme to launch the pump light without significant loss of brightness (which influences which pump sources and DC fibers can be used);
Obstruction of fiber endsxe2x80x94whether the coupling scheme leaves one or both ends of the DC fiber accessible; access to the fiber ends is critical for launching or coupling out of the signal beam, for mode-stripping (to remove unwanted light propagating in the inner cladding), and for fiber connectorization and splicing.
Compatibility with inner-cladding shapesxe2x80x94a number of inner-cladding shapes are available, including round, square, rectangular, hexagonal, octagonal, and star-shaped; a given coupling scheme may not be applicable to all shapes.
Ease of fabricationxe2x80x94affects the manufacturability of the system; determined by, among other factors, the complexity and alignment sensitivity of the coupling scheme.
Cost.
The above considerations are particularly important for practical applications of DC fiber lasers and amplifiers.
The embedded-mirror side pumping apparatus and method of the invention have several advantages, including:
1) The approach is compact and simple (low parts count).
2) The lack of coupling optics reduces the alignment sensitivity (positional and angular) in comparison with alternative approaches.
3) The compactness, simplicity, and relatively low alignment sensitivity allow rugged and stable packaging of the amplifier.
4) The efficiency of the coupling scheme can provide high net efficiency (electrical-to-optical) for the system.
5) The method is compatible with a variety of pump sources, including fiber-coupled sources, diode bars, and multiple pump sources. In particular, as shown in FIG. 8 (described below), embedded-mirror side pumping enables the use of diode bars without increasing the complexity or the parts count over that required for pumping with a single diode laser. This approach may be used to fabricate a single, high-power amplifier or a multitude of amplifiers sharing a single pump source.
6) It is scalable to high powers. In particular, the capability to directly couple a diode bar to a DC fiber will allow DC fiber lasers and amplifiers to be scaled to very high output powers while maintaining compact and rugged packaging.
7) It leaves the fiber ends completely unobstructed.
8) It is compatible with a variety of inner-cladding shapes.
9) The embedded side-mirror can be fabricated from any glass that will transmit the pump beam (i.e., it does not have to be fused silica); BK-7 is a readily available glass that is appropriate.
10) Fabrication of the mirror is decoupled from fabrication of the notch in the fiber, allowing the two to be optimized independently.
11) Fiber pigtailing of the pump diode(s) or the diode bar is not required.
The prior art with the most similarity to the present invention is V-groove side pumping. In comparison with that technique, embedded-mirror side pumping has the following advantages:
Because the mirror is HR coated, the angular acceptance angle is much larger than that for total internal reflection.
Because the notch cut in the fiber is not used for reflection, the required surface quality is not as high.
The alignment sensitivity is substantially reduced in comparison with the usual implementation of V-groove side pumping using a micro-lens, thus increasing the ease of assembly and the stability.
One embodiment of V-groove side pumping entails HR coating of the V-groove to increase the angular acceptance. Embedded-mirror side pumping achieves this benefit without requiring HR coating of the notch, which vastly simplifies the approach. In particular, fabrication, modification, and repair of the system are all made significantly more practical by decoupling production of the mirror from cutting of the notch.
The fiber does not need to be stripped and recoated. Stripping the fiber reduces its mechanical strength, increasing the likelihood of breakage. In addition, for V-groove side pumping, the adhesive used to bond the stripped fiber to the substrate must additionally provide a high NA for the inner cladding (to guide the pump light); in practice, the NA of the stripped and recoated portion of the fiber is usually less than that of the unstripped fiber, leading to loss of pump light. In the present approach, only a short length of the fiber is stripped on one side (to allow the pump diode to be positioned close to the mirror), and the notch can be cut through any of the jacket and outer cladding that remains on the sides of the fiber; no recoating is required, and the adhesive used to attach the fiber to the substrate adheres to the fiber jacket (i.e., there are no restrictions on the refractive index or optical properties of the adhesive).
Embedded-mirror side pumping provides unidirectional pumping. The approaches to V-Docket groove side pumping shown in FIGS. 1b and 1d employ bidirectional pumping, which requires twice the fiber length to achieve the same pump absorption as unidirectional pumping. This extra fiber length is a disadvantage for many applications, including those that need to minimize nonlinear processes in the fiber (e.g., for high peak- or average-power systems or narrow-linewidth sources), minimize fiber consumption (of expensive specialty fibers), or operate at relatively blue wavelengths within the rare-earth gain curve (for Er3+- or Yb3+-doped fibers).
In an embodiment using a curved mirror, embedded-mirror side pumping allows the use of highly divergent pump diodes (as discussed above). V-groove side pumping does not possess this capability; although in principle a curved, optical-quality notch could be cut into the inner cladding, this approach is prohibitively difficult, and it is not evident in the manner that V-groove side pumping has been practiced or taught to date. Moreover, the main benefit of using a curved surface, improving coupling efficiency for highly divergent pump sources, cannot be realized with that technique because of the limited angular acceptance for total internal reflection.
In summary, the present invention (xe2x80x9cembedded-mirror side pumpingxe2x80x9d) introduces a new method for optically pumping DC fiber lasers and amplifiers using diode lasers, diode bars, or fiber-coupled diode sources. This approach addresses all of the above considerations and does not require compromises or trade-offs among the desired characteristics of a pumping scheme. Specifically, a mirror is embedded within the inner cladding of a DC optical fiber; the pump source is brought into close proximity to the mirror, without intervening optical elements, and the pump light is launched into the inner cladding with high efficiency.
This invention solves the long-standing problem of finding a low-cost and practical methodology for side-pumping of rare-earth-doped fiber amplifiers/lasers with commercially available diode lasers. The invention is useful in a wide variety of applications, including IRCM (Infrared countermeasures), trace gas detection, biological warfare agent detection, LIDAR/LADAR, materials processing, free space communication (e.g. secure communications between two satellites), and in medical applications such as tissue welding and ablation.
Additional features and advantages of the present invention will be set forth in, or be apparent from, the detailed description of preferred embodiments which follows.